Light-sensitive device



Oct. 25, 1932.

Current Ac. Current E, D. WILSON LIGHT SENSITIVE DEVICE Filed April 25. 1930 Ga. 5 Filled Highest VaIzaye Low Voltage Currerrf High Va cl/um Voltage Vacuum All Vahajes 4 Above Sntarafin;

AC. Curr-enl- Freguency INVENTOR l Earl D. Wilson ATTORNEY Patented Oct. 25, 1932 UNITED STATES PATENT OFFICE EARL D. WILSON, OF WILKINSBURG, PENNSYLVANIA, ASSIGNOR TO WESTINGHOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION OF PENNSYLVANIA LIGHT-SENSITIVE DEVICE Application filed April 23,

This invention relates to light-sensitive devices and especially to gas-filled phototubes adaptedcto be operated at the ionizing potential of the gas.

An object of the invention'is to operate a gas-filled tube in a system in which highvacuum phototubes have heretofore been used. More specifically, it is an object of the invention to operate a gas-filled phototube at the ionizing potential of the gas therein and to obtain thereby the operating characteristics of high-vacuum phototubes. 7

Commercial phototubes have heretofore been rated to be operated at 90 volts potential drop across the electrodes and load resistance. These tubes have been marketed in two distinct classes. The first class is a highvacuum tube which is operated in the absence of all gas. These high-vacuum tubes are especially adapted for linear measurements, .as the current is proportional to the light falling on the cell. Consequently, these tubes heretofore have been used in all calibrated photometric instruments using phototubes. The current output of a high-vacuum tube is constant, in respect to voltage, provided the voltage is greater than a certain minimum value. In other words, the current-voltage curve is substantially a horizontal line after the first initial rise. cult to make consistently sensitive vacuum tubes in quantity production.

Tubes of the other class are gas-filled, the gas being, generally, argon, at a pressure of 100 to 125 microns. At its rated potential of 90 volts, the gas-filled tube is extremely sensitive and, therefore, is used in sound moving-pictures and also in the detection of small quantities of light.

The gas-filled tube, at its rated voltage, is approxlmately ten times as sensitive to light as the vacuum tube. However, the gas-filled tube, at its rated voltage, decreases ,in efi'ective sensitivity as the frequency of modulation of the incident light increases.

The vacuum tube, on the other hand, is

equally effective at all frequencies. The cur rent in a gas-filled tube, at its rated voltage, is critically dependent on the actual value of the voltage, while, as explained above, in 2.

However, it is sometimes difli- 1930. Serial No. 446,515.

high-vacuum tube, the current is independent of the voltage, after a saturated condition is reached.

In view of these different characteristics of the vacuum and gas-filled phototubes, it has been customary to use either the one or the other according to the operating character'- istics that were desired. Accordingly, two distinct sets of tubes were required in any upto-date laboratory using phototubes for various purposes. This invention, however, contemplates a gas-filled tube that can be used in place of the high-vacuum tube,in addition to'its use as an ordinary gas-filled tube.

Other objects of my invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawing, in which Figure l is a view, partly in perspective, partly in section and partly diagrammatic, of a phototube and connections thereto;

Fig. 2 is a curve illustrating the voltagecurrent characteristic of a gas-filled tube;

Fig. 3 is is a curve illustrating the voltagecurrent characteristics of a high-vacuum phototube;

Fig. 4 is a curve illustrating the currentfrequency characteristic of a gas-filledtube at various voltages; r

Fig. 5 is a curve illustrating a current-frequency characteristic of a high-vacuum tube at various voltages.

In Fig. 1, is disclosed a phototube having a gas-tight container 10, generally of transparent glass, supported on a base 11. Extending through the press 12 of the tube are at least two conductors 13 and 14. To the conductor 13 is attached the cathode 15, preferably in the form of a semi-cylindrical plate attached, at one edge, to the supporting connection 13 and, at the other edge, to an auxiliary support 16. The connection 14 preferably extends up through the axis of the cylindrical cathode l5 and acts as an anode 17. To the upper end of the anode 17 is preferably attached a disc 18 containing a pellet 19 of caesium that has been flashed in the manufacture of the tube to coat the inner surface of the cathode 15 with caesium so that the electrons will be emitted therefrom under the influence of light falling upon this inner surface of the cathode. An electrical pressure applied across the cathode 15 and anode 17 will drive the electrons to the anode, with the result that current will flow from the anode 17 to 'the cathode 15 and thence, through external connections, to the anode 17. The interior of the glass tube 10 is filled with a gas, preferably argon, but, instead of being at the usual pressure, of 100 to 125 microns, this gas is preferably from 75 to 90 microns of pressure.

The tube has'the external connectors 20 and 21 to which any desired system may be attached. As disclosed in Fig. 1, a B voltage supply 22 is connected to a potentiometer 23, and a variable tap 24 on the potentiometer is connected to the terminal 20. From the other terminal 21 extends a connection to an amplifier system represented by the ampli fier tube 25.- This system may include any number of such tubes and may be attached to any desired indicating or recording device. 'The return connection from the output circuit is represented by the conductor 26. A by-pass condenser 27, a load resistance 28, a grid condenser 29 and a grid leak 30 are suitably inserted in the circuit, as disclosed in the drawing. It is obvious that any suitable circuit may be attached to the terminals 20 and 21 of the tube, but such circuits necessarily include an electrical pressure, such as that provided by the B supply 22 in Fig. 1, for driving the electrons from the cathode to the anode, when light falls on the cathode 15.

In Fig. 2 is disclosed a voltage-current curve for a gas-filled tube such as has been described in connection with Fig. 1. It will be noted that the curve rises sharply to the point 31, then has a flat portion to the point 32 and then begins to curve sharply upward. The rated voltage of a gas-filled tube is generally upon the steep part of the curve. With a high gas pressure inside of the tube, the flat portionof the curve between the points 31 and 32, will be very much shorter and the curve will slope upward more quickly after the point 31 is reached. However, by having the pressure in the tube at from 75 to 90.

microns of pressure, instead of the normal 100 to 125 microns, I have provided the longer fiat portion on the curve. This fiat'por'tion is substantially at the ionizing potential of the gas which is represented by the dotted line 33. For argon gas, this ionizing potential is 15.1 volts. The flat portion 3132 of the curve represents a voltage range from 13 to 17 volts which is designated as being substantially at the ionizing potential of the gas in the claims appended hereto.

In Fig. 3 is disclosed the voltage-current characteristic curve for a high-vacuum tube. It will'be noted that the curve quickly rises to a saturated condition and then becomes substantially flat. This indicates that the high-va'cuum-tube current is independent of the voltage after the saturation point is reached.

In Fig. 4 is illustrated the dynamic characteristic of a gas-filled tube with various voltages. These dynamic characteristics refor to the relative alternating-current response for agiven frequency of modulation of the incident light. It will be noted that, at the higher voltages, the response decreases as the frequency increases. However, at a low voltage which has been found to be substantially the ionizing potential of the gas, such as that illustratd by the dotted line 33 in Fig. 2, the response is constant for practically all frequencies.

In Fig. 5, the curve illustrates the fact that the alternating-current response for all voltages above saturation is independent of the frequency in a high-vacuum tube.

A study of the curves of Figs. 2 through 5 will disclose that a gas-filled tube, such as I have described in connection with Fig. I of the drawing, if operated at the ionizing potential of the gas, will give the straightline characteristics of the high-vacuum tube. According to my discovery, therefore, the gas tube of Fig. 1 can take the place of a high-vacuum tube, provided it is operated at substantially the ionizing potential of its gas. Therefore, it will not be necessary to provide tubes of two different types for use in the various devices that have heretofore required.

these two ty es.

Although have shown and described certain specific embodiments of my invention, I am fully aware that many modifications thereof are possible. My invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. In combination, a light-sensitive device comprising a gas-tight container having a transparent portion, a plurality of electrodes in said container, a light-sensitive surface on one of said electrodes, a gas in said container, said gas being at a pressure of substantially 75 to 90 microns and an electrical otential applied across two of said electro es at an ionizable potential of said gas.

2. In combination, a light-sensitive device comprising a gas-tight container having a transparent portion, a plurality of electrodes in said container, a light-sensitive surface on one of said electrodes, a gas in said container, and a source of electrical pressure applied to said light-sensitive surface electrode and another of said electrodes, said electrical pressure being substantially at the ionization potential of said gas.

3. In combination, a light-sensitive device comprising a gas-tight container having a transparent portion, a plurality of electrodes in said container, a light-sensitive surface on one of said electrodes, a gas in said container, said gas being at a pressure of substantially 75 to 90 microns, and a source of electrical ressure apglied to said light-sensitive-surace electro e and another of said electrodes, said electrical pressure being substantially at the ionization gotential of said gas.

In testimony w ereof, I have hereunto sub- 10 scribed my name this 18th dag of April, 1930. EARL WILSON. 

